You will get a coin multiplier for collecting a lot of coins in a row. The longer you can maintain your streak, the higher the multiplier will increase. It is somewhat difficult to get it past X2. In order to do so, you must collect coins very quickly. After a row of coins has finished, get to the next as fast as possible. You only have about two seconds before the multiplier starts to decay, at which point filling it up again will only reset the multiplier at its current level. Save your coins to buy new cars instead of spending them on upgrading temporary power ups.